Improve tup config;

- Add config folder that contains tup config files.  There is a default
  config added to source control, but everything else in that folder is
  gitignored so you can add your own custom configurations.
- Remove and gitignore tup.config.
- This results in the following setup:
  - You can now create a tup.config symlink that points at the config
    you want to use.
  - Or, you can use the `tup variant` command to manage multiple build
    configurations at the same time (e.g. debug, release, wasm).

.gitignore

@@ -1,6 +1,7 @@
 .tup
 .obj
-*.config
+tup.config
+config/*
 /build.sh
 /watch.sh
 /debug.sh